Hood: Outlaws & Legends

This one can happen as a fluke in the middle of battle, or it can be a major PITA. Fortunately, this can be done on state AI, as opposed to hoping you can line up two enemy players.

For me, I did it on graveyard at control point B.
I made sure I had a smoke grenade already, but there is a cache down by B if you couldn't find one.
I got the attention of two guards in the courtyard, and ran down to B as they gave chase. Once they were relatively close together, I smoked them and shot them up a few times.
NOTE: just trying for double headshots on them right away won't work. Whittle them down with melee or a couple shots, first.

Once you've got their health down a bit, make sure you have enough bolts for a burst shot, and line up the heads. It may take a few attempts for you to make the burst work. It took me twelve games trying to get this due to various factors... so, good luck.